Synthetic antiferromagnet structures for use in MTJs in MRAM technology

ABSTRACT

A magnetic tunnel junction (MTJ), which is useful in magnetoresistive random access memories (MRAMs), has a free layer which is a synthetic antiferromagnet (SAF) structure. This SAF is composed of two ferromagnetic layers that are separated by a coupling layer. The coupling layer has a base material that is non-magnetic and also other materials that improve thermal endurance, control of the coupling strength of the SAF, and magnetoresistance ratio (MR). The preferred base material is ruthenium and the preferred other material is tantalum. Furthering these benefits, cobalt-iron is added at the interface between the tantalum and one of the ferromagnetic layers. Also the coupling layer can have even more layers and the materials used can vary. Also the coupling layer itself can be an alloy.

FIELD OF THE INVENTION This invention relates to magnetic tunneljunctions (MTJs), and more particularly to MTJs that use syntheticantiferromagnet (SAF) structures for the free layer. RELATED ART

Magnetoresistive random access memories (MRAMs) are known to have manybenefits such as being fast, non-volatile, and high density. There aredifficulties, however, in making MRAMs in a manufacturable manner. Oneof the difficulties that have been addressed is the difficulty withwriting the MRAM cells reliably. This has been effectively addressedwith a toggle bit which changed both the manner of writing and thestructure of the free layer over that previously used. This particularsolution typically uses a SAF structure for the free layer. Solving thewrite problem using the SAF structure for the free layer then changedthe considerations required for improving the MRAM bit cell, and moreparticularly the MTJ portion of the MRAM bit cell. One of the continuingdesires with MRAM is to improve the magnetoresistance (MR) ratio, whichis the ratio of the change in resistance between the two logic states tothat of the low resistance state. Replacing a single free layer with aSAF free layer can reduce the MR. Since the signal available to thesense circuitry is proportional to the MR, an improvement that increasesMR in an MTJ having a SAF free layer will result in an improvement insensing speed. Another issue is the ability to control theantiferromagnetic coupling strength of the free layer SAF. This couplingneeds to be controlled so that the current for writing is maintainedwithin an acceptable range. Another issue is endurance, especiallythermal endurance. MTJ materials tend to have higher sensitivities toelevated temperatures than the materials used in some semiconductorprocesses. In particular, SAF materials have a failure mode at elevatedtemperatures that causes a degradation of the antiferromagnetic couplingstrength.

Thus, there is an ongoing need for MRAM development to improve one ormore of thermal endurance, MR ratio, and control of the write currents.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not limitedby the accompanying figures, in which like references indicate similarelements, and in which:

FIG. 1 is a cross section of a magnetic tunnel junction (MTJ) for use inan MRAM according to an embodiment of the invention;

FIG. 2 is a cross section of a portion of the MTJ of FIG. 1 according tothe embodiment of the invention;

FIG. 3 is a cross section of a portion of the MTJ of FIG. 1 according toan alternative embodiment of that shown in FIG. 2; and

FIG. 4 is a cross section of a portion of the MTJ of FIG. 1 according toanother alternative embodiment of that shown in FIG. 2.

Skilled artisans appreciate that elements in the figures are illustratedfor simplicity and clarity and have not necessarily been drawn to scale.For example, the dimensions of some of the elements in the figures maybe exaggerated relative to other elements to help improve theunderstanding of the embodiments of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In one aspect a magnetic tunnel junction (MTJ), which is useful inmagnetoresistive random access memories (MRAMs), has a free layer whichis a synthetic antiferromagnet (SAF) structure. This SAF is composed oftwo ferromagnetic layers that are separated by a coupling layer. Thecoupling layer has a base material that is non-magnetic and also othermaterials that improve thermal endurance, control of the couplingstrength of the SAF, and MR. The preferred base material is rutheniumand the preferred other materials are tantalum and a cobalt-iron alloy.This is better understood by reference to the drawings and the followingdescription.

Shown in FIG. 1 is an MTJ 10 comprising a top electrode 12, a free SAF14 immediately under top electrode 12, a tunnel barrier 16 immediatelyunder SAF 14, a fixed SAF 18 immediately under tunnel barrier 16, apinning layer 20 immediately under fixed SAF 18, a seed layer 22immediately under layer 20, and a base electrode 24 immediately underseed layer 22. Free SAF 14 comprises a ferromagnetic layer 26immediately under top electrode 12, a coupling layer 28 immediatelyunder ferromagnetic layer 26, and a ferromagnetic layer 30 immediatelyunder coupling layer 28. Fixed SAF 18 comprises a ferromagnetic layer 32immediately under tunnel barrier 16, a coupling layer 34 immediatelyunder ferromagnetic layer 32, and a ferromagnetic layer 36 immediatelyunder coupling layer 34. Ferromagnetic layers 32 and 36 are preferablycomprised of alloys that include cobalt and iron. Except that couplinglayer 28 comprises a combination of materials, preferably ruthenium andtantalum and ferromagnetic layer 30 preferably includes cobalt and ironat the interface with coupling layer 28, MTJ 10 is known to one ofordinary skill in the art. It is known to those skilled in the art thatthe fixed SAF 18 can be replaced by other structures that provide amagnetically fixed layer 32 in contact with the tunnel barrier 16, suchas a single pinned ferromagnetic layer, such that the magnetic momentvector of the fixed layer 32 does not move substantially in the appliedfields used to switch the free layer.

Shown in FIG. 2 is SAF 14 comprising ferromagnetic layers 26 and 30, aninsertion layer 41, and a coupling layer 28 comprised of a base layer 38immediately under ferromagnetic layer 26 and an insertion layer 40immediately under base layer 38. Insertion layer 41 is betweenferromagnetic layer 30 and insertion layer 40. Insertion layer 41,having the ferromagnetic materials of iron and cobalt, functionsmagnetically as part of ferromagnetic layer 30. Insertion layer 41 ispreferably cobalt iron alloy but may be other materials instead, such ascobalt or iron. Base layer 38 preferably comprises ruthenium but couldalso comprise another base material such as rhodium, iridium, andosmium. Base material as used herein means a material that itself can besufficient to provide the needed antiferromagnetic coupling between thetwo ferromagnetic materials in a SAF. Insertion layer 40 is preferablytantalum and is inserted to improve the properties of SAF 14. Insertionlayer 41 is preferably cobalt-iron and is also inserted to improve theproperties of SAF 14. Ferromagnetic layers 26 and 30 are preferablynickel-iron alloy with 16 to 20 atomic percent iron, and more preferablyabout 18 atomic percent iron.

Insertion layer 41 is preferably deposited by ion beam deposition usinga cobalt-iron target, but another process could be used. One example ofan alternative is magnetron sputtering. This insertion layer 41 ispreferably deposited directly on ferromagnetic layer 30. Insertion layer40 is then deposited directly on insertion layer 41 by ion beamdeposition using a tantalum target but another process, such asmagnetron sputtering, could also be used. The tolerance is held as tightas possible, but the thickness range of coupling layer 28 can be 6-10angstroms. Base layer 38 is preferably 6.0 to 6.5 angstroms andinsertion layer 40 is preferably deposited to a thickness of 2.5angstroms. Insertion layer 41 is deposited to a thickness of preferably2.5 angstroms that is held as tight as possible but the range can be1.5-5 angstroms. These dimensions are based on using ruthenium as thebase material having a coupling peak centered around 7-8 angstroms.Another peak could be used that would significantly affect thosedimensions. With insertion layers 40 and 41 being deposited tosub-atomic thicknesses, insertion layers 40 and 41 may appear as asingle alloyed layer. Thus, it may be difficult to actually distinguishlayers 40 and 41 in a finished MTJ such as MTJ 10.

Base layer 38 provides sufficient coupling between ferromagnetic layers26 and 30 to achieve antiferromagnetic alignment, and the addition ofinsertion layers 40 and 41 improves certain characteristics of theresulting SAF 14. For example, one improvement is in the thermalendurance. The temperature at which failure of SAF 14 occurs is higherthan if ruthenium alone is used. Another improvement is in the controlof coupling strength between ferromagnetic layers 26 and 30. Thethickness of base layer 38 is difficult to keep from varying overprocess variations. The coupling strength varies with this thickness.With insertion layers 40 and 41 added, the rate of change of couplingstrength varies less with changes in base layer thickness resulting inincreased control of the coupling strength. Yet another improvement isan improvement in the MR ratio of MTJ 10. For example, if the MR ratioprior to the addition of insertion layers 40 and 41 was about 30percent, then it is observed to increase by 3 percentage points due tothe insertion of insertion layers 40 and 41, which is about 10 percentimprovement. There may be circumstances where it would be beneficial toleave off one of the added layers 40 and 41.

Shown in FIG. 3 is an alternative SAF 42 for replacing SAF 14 in FIG. 1.SAF 42 comprises a ferromagnetic layer 46 immediately under topelectrode 12, a coupling layer 48 immediately under ferromagnetic layer46, a ferromagnetic layer 50 immediately under coupling layer 48. Inthis case coupling layer 48 is an alloy of a base material and anothermaterial that improves thermal endurance and control of couplingstrength. Examples of the base material are ruthenium, rhodium, iridium,and osmium. The added materials that are added to form an alloy arechosen from boron, aluminum, carbon, tantalum, niobium, molybdenum,zirconium, and hafnium. The preferred combination is ruthenium and boronbecause it demonstrates better endurance under high temperatureannealing compared to a SAF having the same coupling strength achievedwith ruthenium alone. Coupling layer 48 is preferably in the range of6-10 angstroms. Again, these dimensions are based on using ruthenium asthe base material having a coupling peak centered around 7-8 angstroms.Another peak could be used that would significantly effect thosedimensions.

Shown in FIG. 4 is another alternative SAF 60 for substitution with SAF14 of FIG. 1. SAF 60 comprises ferromagnetic layer 64 immediately undertop electrode 12, a coupling layer 62 immediately under ferromagneticlayer 64, and a ferromagnetic layer 66 immediately under coupling layer62. In this case coupling layer 62 is a composite of multilayerscomprising layer 78 on ferromagnetic layer 66, layer 76 on layer 78,layer 74 on layer 76, layer 72 on layer 74, layer 70 on layer 72, andlayer 68 on layer 70. This multilayer configuration is alternating basematerial and added materials to improve characteristics of SAF 60 andthus MTJ 10. In these layers 68, 72, and 76 are base material and layers70, 74 and 78 are added materials. This is an example of six layers butthere could be fewer or more. Added materials may be one of nickel-iron,cobalt-iron, tantalum, and aluminum. These have been shown to providethe benefit of control of the coupling strength. The total thickness ofcoupling layer 62 should be in the range of 6-10 angstroms. In thisexample of 6 layers, each of layers 68-78 should be in the range of1-1.7 angstroms to achieve the desired thickness for coupling layer 62.At these small dimensions it may be difficult to discern the individuallayers. This multilayer approach has been shown to give more control inselecting the coupling strength. The preferred materials are rutheniumfor the base material and tantalum as the added material.

In the foregoing specification, the invention has been described withreference to specific embodiments. However, one of ordinary skill in theart appreciates that various modifications and changes can be madewithout departing from the scope of the present invention as set forthin the claims below. Accordingly, the specification and figures are tobe regarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofpresent invention.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus.

1-21. (canceled)
 22. In an MTJ comprising: a first electrode and asecond electrode; a tunnel barrier between the first and secondelectrodes; a free synthetic antiferromagnet (SAF) between the firstelectrode and the tunnel barrier; and a fixed layer between the tunnelbarrier and the second electrode; a method of forming the free SAF,comprising: forming a first ferromagnetic layer over the tunnel barrier;depositing tantalum over the first ferromagnetic layer; depositing abase material over the tantalum; and forming a second ferromagneticlayer over the base material.
 23. The method of claim 22, furthercomprising depositing a third ferromagnetic layer over the firstferromagnetic layer before depositing the tantalum over the firstferromagnetic layer, the third ferromagnetic layer comprising at leastone of cobalt and iron.
 24. The method of claim 22, wherein the basematerial comprises one of rhodium and ruthenium.
 25. The method of claim22, wherein the first ferromagnetic layer comprises a nickel-iron alloy.26. The method of claim 25, wherein the nickel-iron alloy is about 16 to20 atomic percent iron.
 27. The method of claim 26, wherein thenickel-iron alloy is about 18 atomic percent iron.
 28. (canceled)